Introduction

Except for rock samples brought back from
the Moon by Apollo astronauts, cosmic ray
particles that reach the atmosphere, and
meteorites that fall to Earth, the only
information about objects in space comes to
Earth in the form of electromagnetic
radiation. How astronomers collect this
radiation determines what they learn from it.
The most basic collector is the human eye.
The retina at the back of the eye is covered
with tiny antennae, called rods and cones,
that resonate with incoming light. Resonance
with visible electromagnetic radiation
stimulates nerve endings, which send
messages to the brain that are interpreted
as visual images. Cones in the retina are
sensitive to the colors of the visible
spectrum, while the rods are most sensitive
to black and white.

Until the early 1600s, astronomers had only
their eyes and a collection of geometric
devices to observe the universe and
measure locations of stellar objects. They
concentrated on the movements of planets
and transient objects such as comets and
meteors. However, when Galileo Galilei
used the newly invented telescope to study
the Moon, planets, and the Sun, our know-ledge
of the universe changed dramatically.
He was able to observe moons circling
Jupiter, craters on the Moon, phases of

Venus, and spots on the Sun. Note: Galileo
did his solar observations by projecting light
through his telescope on to a white
surface--a technique that is very effective
even today. Never look at the Sun
directly with a telescope!

Galileo's telescope and all optical
telescopes that have been constructed since
are collectors of electromagnetic radiation.
The objective or front lens of Galileo's
telescope was only a few centimeters in
diameter. Light rays falling on that lens
were bent and concentrated into a narrow
beam that emerged through a second lens,
entered his eye, and landed on his retina.
The lens diameter was much larger than the
diameter of the pupil of Galileo's eye, so it
collected much more light than Galileo's
unaided eye could gather. The telescope's
lenses magnified the images of distant
objects three times.

Since Galileo's time, many huge telescopes
have been constructed. Most have
employed big mirrors as the light collector.
The bigger the mirror or lens, the more light
could be gathered and the fainter the source
that the astronomer can detect. The famous
5-meter-diameter Hale Telescope on Mt.
Palomar is able to gather 640,000 times the
amount of light a typical eye could receive.
The amount of light one telescope receives
compared to the human eye is its light